Observability results related to fractional Schr\"odinger operators

We establish observability inequalities for various problems involving fractional Schr"odinger operators $(-\mathrm{\Delta }{)}^{\alpha \mathrm{/}2}+V$, $\alpha >0$, on a compact Riemannian manifold. Observability from an open set for the corresponding fractional Schr"odinger evolution equation with $\alpha >1$ is proved to hold as soon as the observation set satisfies the Geometric Control Condition; it is also shown that this condition is necessary when the manifold is the $d$-dimensional sphere equipped with the standard metric. This is in stark contrast with the case of eigenfunctions. We construct potentials on the two-sphere with the property that there exist two points on the sphere such that eigenfunctions of $-\mathrm{\Delta }+V$ are uniformly observable from an arbitrarily small neighborhood of those two points. This condition is much weaker than the Geometric Control Condition, which is necessary for uniform observability of eigenfunctions for the free Laplacian on the sphere. The same result also holds for eigenfunctions of $(-\mathrm{\Delta }{)}^{\alpha \mathrm{/}2}+V$, for any $\alpha >0$.